This is to be a double-blind placebo-controlled crossover study of methylphenidate and fenfluramine in 30 mentally retarded children with hyperactivity (Attention Deficit Hyperactivity Disorder). Methylphenidate will be given in standardized doses of 0.4 mg/kg/day, whereas three doses of fenfluramine (1.0, 1.5, 2.0 mg/kg/day) will be compared. Each drug condition will be administered for two weeks. All children will be assessed for changes on standardized parent and teacher behavior rating scales, tests of cognitive performance, an independent academic task, systematic behavior observations, heart rate and blood pressure, and an extensive listing of side effects. The data will be analyzed to (1) evaluate the clinical (behavioral) effects of each drug, (2) compare drug effects on learning performance, (3) determine if one of the fenfluramine doses is superior for clinical and/or cognitive outcome, and (4) determine whether certain compelling subject characteristics are helpful in predicting drug response.